This invention relates to a multi-stage process for preparing alpha-olefins from light olefins. In an initial stage, light olefins such as propylene, butylene, and the like, are converted in the presence of a shape selective crystalline zeolite catalyst to higher olefins. At least a portion of these higher olefins are then reacted with alpha-olefin in the presence of a metathesis (disproportionation) catalyst to provide alpha-olefins having different numbers of carbon atoms than that of the feed olefins. The alpha-olefin product resulting from the metathesis operation is further processed to provide at least two fractions the lighter of which is recycled as cofeed to the first stage conversion unit.
Conversion of olefins to gasoline and/or distillate products is disclosed in U.S. Pat. Nos. 3,960,978 and 4,021,502 wherein gaseous olefins in the range of ethylene to pentene, either alone or in admixture with paraffins, are converted into an olefinic gasoline blending stock by contacting the olefins with a catalyst bed made up of ZSM-5 zeolite. In U.S. Pat. No. 4,227,992, the operating conditions for the selective conversion of C.sub.3 + olefins to mainly aliphatic hydrocarbons in the gasoline/distillate range is disclosed. In a related manner, U.S. Pat. Nos. 4,150,062 and 4,211,640 disclose a process for converting olefins to gasoline components.
In the process for catalytic conversion of olefins to heavier hydrocarbons by catalytic oligomerization using a medium pore shape selective acid crystalline zeolite, such as ZSM-5 catalyst, process conditions can be varied to favor the formation of hydrocarbons of varying molecular weight. At moderate temperature and relatively high pressure, the conversion conditions favor C.sub.1O + aliphatic product. Lower olefinic feedstocks containing C.sub.2 -C.sub.8 alkenes may be converted; however, the distillate mode conditions does not convert a major fraction of ethylene. U.S. Pat. No. 4,547,612 discloses a continuous process for the catalytic conversion of olefins to lubricant or heavy distillate range compounds in which a light olefin feedstock, e.g., propylene, is combined with a C.sub.5 + olefin stream recovered from previous product effluent.
Olefin metathesis (disproportionation) is a known type of reaction in which one or more olefinic compounds are transformed into other olefins of different molecular weights. The reaction of an olefin with itself to produce an olefin of a higher molecular weight and an olefin of a lower molecular weight can also be referred to as a self-disproportionation. For example, propylene can be disproportionated to ethylene and cis-, and trans-2-butene. Another type of disproportionation involves the cross-disproportionation of two different olefins to form still other olefins. An example would be the reaction of one molecule of 2-butene with one molecule of 3-hexene to produce two molecules of 2-pentene. Another example of a cross-disproportionation involves the reaction of an internally unsaturated olefin with an alpha-olefin to provide two different alpha-olefins, e.g., the reaction of 2,4,4-trimethyl-2-pentene with ethylene to provide equimolar amounts of 3,3-dimethyl-1-butene (neohexene) and isobutene as shown in Banks, "Olefin Metathesis: Technology and Application", Applied Industrial Catalysis, Vol. 3, Chapter 7, pp. 215 et seq., Leach, ed. (1984).
Among the catalysts that have been developed for olefin metathesis are those comprising inorganic refractory materials containing a catalytic amount of at least one of molybdenum oxide and tungsten oxide. Other olefin metathesis reactions and catalyst compositions therefor are described in U.S. Pat. Nos. 3,883,606; 3,915,897; 3,952,070; 4,180,524; 4,431,855; 4,499,328; 4,504,694; 4,517,401; and, 4,547,617, among others.
The terms "disproportionation" and "metathesis" as used herein mean the conversion of an olefinic hydrocarbon feed to a mixture of product olefinic hydrocarbons having different numbers of carbon atoms than the feed olefins.